With high specific strength and high specific modulus, fiber reinforced composite materials containing carbon fiber or aramid fiber as reinforcement fiber have been used in recent years for manufacturing of structural members of aircraft and automobiles, tennis rackets, golf shafts, fishing rods and other sports goods, as well as general industrial products.
The production processes commonly used for these fiber reinforced composite materials include the curing of a prepreg, i.e. a sheet-like intermediate material produced by impregnating reinforcement fiber with uncured matrix resin, and the resin transfer molding method which consists of placing reinforcement fiber in a mold, injecting a liquid resin in it to prepare an intermediate material, and then curing it. Of these production processes, the prepreg-based method commonly comprises stacking several prepreg sheets and heating and pressing them to mold a fiber reinforced composite material. From the viewpoint of productivity such as processability, thermosetting resins, epoxy resin in particular, have been commonly used as the matrix resin for these prepregs.
As demands increase, there has been much call in recent years for materials with reduced weight and increased strength for structural members of aircraft and automobiles in particular. Accordingly, the epoxy resin used as matrix resin is required to have high heat resistance.
In general, resin compositions with a high glass transition temperature, Tg, cure at a high temperature, and these resin compositions commonly contain much volatile constituents that volatilize when exposed to a high temperature during curing or molding processes. If a large amount of volatile matter volatilizes during curing, the volatile matter will be gasified when the material is used, for instance, as the surface layers of honeycomb panels. Consequently, it will be entrapped in the closed spaces of the honeycomb plates, and will expand there to cause damage to the adhesion between the surface layer and the honeycomb core. Such volatile matter can also form voids when laminated prepreg sheets are cured in an autoclave, leading to fiber reinforced composite materials with a decreased strength.
To provide a highly heat resistant epoxy resin composition with a small volatile matter content, a technique has been proposed to combine a polyfunctional epoxy resin and polyisocyanate or other appropriate polymers (see Patent document 1). This proposal, however, does not refer to strength of the fiber composite material produced by curing laminated prepreg plates.
To provide fiber reinforced composite materials with high strength, it is necessary for the reinforcement fiber to have enhanced strength and an increased volume fraction (high Vf) of fiber. A method to produce a high strength reinforcement fiber has been proposed conventionally (see Patent document 2). This proposal, however, does not refer to the strength of the resulting fiber reinforced composite material. In general, as the reinforcement fiber used has a higher strength, it tends to be more difficult to allow the component fiber to show its inherent strength. If the reinforcement fiber has an improved strand strength, for instance, it will be difficult to produce material with a sufficiently increased tensile strength. Instead, the rate of contribution to tensile strength, which is defined as (tensile strength of fiber reinforced composite material)/(reinforcement fiber's strand strength×fiber volume content)×100, tends to decrease. If carbon fiber with high strength is available, therefore, there remain technical problems to be solved to allow the strength to contribute to producing fiber reinforced composite materials with increased strength.
It has been known that even if using reinforcement fibers with the same level of strength, the rate of contribution to tensile strength can vary significantly depending on the matrix resin to be combined and the molding conditions to be used. If curing is to be performed at a high temperature of 180° C. or more, in particular, thermal stress takes place during the curing process and remains in the resulting fiber reinforced composite material, preventing it from developing high strength. Thus, studies have been carried out to provide improved matrix resins that can serve to develop adequate tensile strength even when cured at a temperature of 180° C.
It has been known that the use of a matrix resin with an increased tensile elongation at rupture serves to produce a fiber reinforced composite material with an improve rate of contribution to tensile strength. The tensile elongation at rupture of a matrix resin can be increased effectively by decreasing the crosslink density of the matrix resin, but a decrease in the crosslink density can reduce the heat resistance of the resulting fiber reinforced composite material. This limits the effective range of the blending ratio, posing a problem. To solve the problem, it is proposed that a high rate of contribution to tensile strength can be achieved when the tensile elongation at rupture and the fracture toughness, KIc, meet a specific relation (see Patent document 3). If a large amount of thermoplastic resin or a rubber component is added to the matrix resin with the aim of improving the fracture toughness, KIc, however, the viscosity will generally increase, leading to deterioration in the processability and handleability in the prepreg production process.